The Tale of the Camera Obscura
The term camera obscura (Latin for “dark chamber”) was introduced in the 17th century to describe the use (already known to the ancients) of a small light-tight room or box fitted with a small hole on 1 side. The hole allowed the inverted image of a bright scene or bright object to be projected onto the opposite wall or side of the box, where it might be conveniently viewed or studied. The earliest known drawing of such a device is shown in Figure I-1, where it is being used to view a solar eclipse.
Experience with such devices leads to 3 important observations:
The projected image is inverted.
The “depth of field” of the projected image is superb: objects are simultaneously in focus at all distances, from foreground objects to distant hills and even astronomical objects (Figure I-2).
The image is very dim.
The inverted image clearly arises from the straight-line propagation of light rays originating from each point in the original object, with the pinhole acting like a sort of fulcrum (Figure I-3).
The great depth of field is due to the small aperture, which allows light rays from each object to reach only a very small region in the image plane.
Figure I-1 Earliest known depiction of a camera obscura.
(From De Radio Astronomica et Geometrica; 1545.)
Figure I-2 Camera obscura (pinhole camera) image. Notice the great depth of field, with both the rocks on the foreground and the mountains in the background simultaneously in sharp focus.
(Courtesy of Mark James.)
Figure I-3 Image formation in a camera obscura. An inverted image forms when rays of light from points of the original object (eg, A, D, B) follow straight paths through the pinhole (C) to the corresponding points (eg, b, d, a, respectively) in the image on the far wall.
Of course, the small aperture also greatly limits the amount of light that is available to form the image, which explains why it is so dim.
To obtain a brighter image, suitable for the activation of a detector (such as a photographic plate, a CCD chip, or the retina), it is necessary to enlarge the pinhole aperture to admit more light. Unfortunately, enlarging the aperture also allows the rays of light emanating from each point of the source object to form a cone of light, which illuminates a proportionally larger disc on the image plane. These “blur circles” (or perhaps more generally, “blur ellipses”) smear out the image, resulting in substantial blurring (Figures I-4, I-5).
To recover a sharp image while retaining the image intensity afforded by a larger aperture, it is necessary to recombine the light rays that originate from each point of the source object so that they will converge on a single image point. This can be accomplished by placing a suitable lens in the aperture (Figure I-6).
However, the strategy of using a lens to recover the sharpness of images by enlarging the aperture necessarily sacrifices the depth of field obtained with the simple pinhole. Though the lens will simultaneously bring to focus rays of light from different points in the source object, it can do so only for source points at the same distance from the lens (Figure I-7).
Figure I-4 Enlarging the pinhole in a camera obscura results in larger blur-circle images of each point of the original object.
Figure I-5 Effect of pinhole size (as indicated schematically by the circles at the top) on image sharpness in a camera obscura.
Figure I-6 Recovering a sharp image by placing a lens in the aperture of a camera obscura after the aperture has been enlarged. The same lens simultaneously recombines the rays of light from each point of the source object to land at a single point in the image plane.
These observations about image formation in simple cameras essentially summarize the entire challenge of basic optics: to form sharp images from beams of light defined by apertures of finite size to provide sufficient brightness for the application at hand. The rules for the proper selection of lenses for this purpose are discussed in the next section.
Figure I-7 Reduced depth of field in images obtained with lenticular optics. Even in this case, smaller aperture (larger f/ numbers) result in greater depth of field.
(Courtesy of Scott E. Brodie, MD, PhD.)
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.